This invention relates to antennas, and, more particularly, to a structurally simple polarimetric antenna of particular use in the sub-microwave, microwave, and millimeter wave frequency ranges.
Microwaves are electromagnetic waves having frequencies of from about 1 GHz (gigahertz) to about 30 GHz. Millimeter waves have even higher frequencies of from about 30 GHz to about 300-500 GHz. Microwaves and millimeter waves are often used to transmit electromagnetic energy in a variety of applications, including radar and communications. Microwaves and millimeter waves can either be radiated through free space from place to place, or carried along a conductive path.
For radar tracking applications, in which radiated electromagnetic energy is used to track objects such as spacecraft, the microwave or millimeter wave electromagnetic energy is generated and propagated through a waveguide to an antenna. The energy is then radiated from the antenna, operating in a transmitting mode, through free space to the object being tracked. A portion of the energy reflects from the object back to the antenna, now operating in a receiving mode, and is received. The electromagnetic energy is transmitted back through the waveguide to a receiver and analyzed.
It has long been known that various transmitted and received modes of such electromagnetic energy can be used to provide information about the path and speed of the object being tracked. A tracking algorithm typically specifies the nature of the electromagnetic energy pattern to be radiated toward the object being tracked and, based upon such a radiated pattern, provides a procedure for analyzing the reflected pattern. For example, in one tracking algorithm developed by Cook and Lowell and described in "The Autotrack System," The Bell System Technical Journal, July 1963, pages 1283-1307, the antenna must transmit a circularly polarized sum pattern and receive sum vertical (.SIGMA..sub.v), sum horizontal (.SIGMA..sub.h) and difference (.DELTA.) patterns.
Once a tracking algorithm is adopted, an antenna and antenna feed must be designed to permit the selected electromagnetic signals to be radiated and received. For complex transmitting and receiving requirements, particularly where circularly polarized patterns must be transmitted and both polarization components received, the conventional approach has been to use complex antenna feeds. Such mechanically complex antenna feed systems tend to be rather costly. Manufacturing procedures are complicated and labor intensive, particularly for feeds designed for use at millimeter-wave frequencies.
There is an ongoing need for improved microwave and millimeter wave polarimetric antennas for general use, and particularly for use in tracking antennas and antenna feed systems. The antennas must be capable of transmitting and receiving the required types of electromagnetic energy patterns, and should be less complex than those already available. The present invention fulfills this need, and further provides related advantages.